Wave mode converter



March 1-0, 1970l E. R. NACELBERG WAVE MODE CONVERTER Filed Dea. 1e. 1968($338930 Nl) ll BCIIIDBAVM NI SHGOW mamas a'loNv asvud '.)NVENTOR E. R.NAGELBERG y ATTORNEY United States Patent O U.S. Cl. 333--21 2 ClaimsABSTRACT OF THE DISCLOSURE A straight waveguide structure is disclosedwith a portion of the structure having a first inside diameter and theremainder of the structure having a second inside diameter to form acircularly symmetric discontinuity. This structure includes an annularlyshaped dielectric member in the larger diameter portion immediatelyadjacent to the discontinuity. In use, the structure converts energy inthe TE11 mode into energy in the TE11TM11 dual mode.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to electromagnetic wave mode converters which convert energy inthe TE11 mode into energy in the TE11TM11 dual mode.

Description of the prior art One very simple prior art dual modeconverter effects conversion by way of a circularly symmetricdiscontinuity formed by a waveguide structure having a first portion atone inside diameter and another portion at another inside diameter.

In this converter the diameters are chosen so that the smaller diameterportion supports energy in the TE11 mode only, while the larger diameterportion supports energy in the TE11 and the TMu" modes only. When energyin the TEU mode is fed into the smaller diameter portion, some of thisenergy is converted into the TMll" mode by the discontinuity so thatenergy in both modes appears in the larger diameter portion to produce asocalled dual mode. A detailed discussion of this appears in ModeConversion in Circular Waveguides by E. R. Nagelberg and I. Shefer, BellSystem Technical Journal, vol. 44 (1965), pages 1321 through 1338.

Notwithstanding the highly desirable simplicity of the above-describedconverter, the phase angle between the two modes forming the dual modevaries as a function of frequency to an extent which is undesirable inapplications such as feed lines for low noise antennas.

SUMMARY OF THE INVENTION An object of the present invention is toreduce, in the above-described dual mode converter, the variation withfrequency of the phase angle between the two modes forming the dualmode.

This and other objects of the invention are achieved through the use ofan annularly shaped dielectric member inserted in the above-describeddual mode converter. In particular, this member has an outside diametersubstantially equal to the inside diameter of the larger of the twoinside diameters of the waveguide structure and an inside diametersubstantially equal to the smaller inside diameter. Furthermore, themember is located in the larger diameter portion adjacent to the smallerdiameter portion; in other words, it abuts the discontinuity formed bythe two diameters. As is discussed in greater detail in the followingdescription, this relatively simple and inexpensive addition to thisprior art converter results in a substantial reduction in the variation,with frequency, of the phase angle between the two modes forming thedual mode.

3,500,258 Patented Mar. 10, 1970 Other objects and features of theinvention will become apparent from a study of the following detaileddescription of a specific embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows a cut-away view of an embodiment of the invention; and

FIG. 2 is a graph showing results of measurements performed while usingthe embodiment in FIG. 1.

DISCRIPTION OF THE DISCLOSED EMBODIMENT FIG. 1 shows a cut-away view ofan embodiment of the invention comprising a pair of waveguides 10 and11. Waveguide 10 has an inside diameter which supports electromagneticwave energy in the TE11 mode only, while waveguide 11 has an insidediameter which supports the TE11 and TM11 modes only. Furthermore, oneend of waveguide 10 is electrically joined to one end of waveguide 11 ina conventional coaxial manner. As appreciated by those skilled in theart, this structure may be made out of a single piece of metal. Inaddition, it may be considered as a single waveguide whose diameterthrough one portion is greater than that throughout the rest of thewaveguide. In either case, the conguration of the structure is identicalto that discussed in the above-mentioned Bell System Technical Journalarticle.

Unlike the structure of the previously mentioned article, the embodimentof FIG. l includes an annularly shaped dielectric member 12. This memberhas an outside diameter substantially equal to the inside diameter ofwaveguide 11 and an inside diameter substantially equal to that ofwaveguide 10. Furthermore, this member is iixed in waveguide 11 so as toabut the junction of the two waveguides.

The two arrows shown to the left and right of the embodiment of FIG. 1indicate that when electromagnetic wave energy in the TE11 mode is fedinto smaller diameter waveguide 10, energy in the TE11TM11 dual modeappears in the larger diameter waveguide 11. The conversion of some ofthe TEHJ mode energy into TM11 mode energy occurs as a result of thediscontinuity at the junction. The effect of the dielectric member onthis conversion may be appreciated by considering the followingdiscussion in conjunction with the curves shown in FIG. 2.

FIG. 2 shows a phase angle vs. frequency graph on which appears a pairof lines 13 and 14 derived from measurements made while using theembodiment of FIG. 1. In the embodiment, the inside diameters ofwaveguides 10 and 11 were 2.1 and 2.8 inches, respectively. Furthermore,member 12 was made of polystyrene and had a thickness of 0.454 inch. Asuccession of`discrete-frequency waves in the TE11 mode were fed intowaveguide 10 with the phase angle between the two modes in waveguide 11measured for each input wave. Line 13 is a smooth line drawn through themeasured phase angles. Identical measurements where made with member 12removed so that the structure was identical to the prior art structure.Line 14 is a smooth line drawn through the measurements made in theabsence of member 12.

From FIG. 2, it is believed readily apparent that the presence of member12 reduces the variation of the phase angle with frequency. Thisimprovement is attributed to the effect of the dielectric loading on thephase angle at the junction between the waveguides (conversion occurs atthis junction) and the time delay introduced by member 12.

Measurements were also made using dielectric members of otherthicknesses. The following effects were noted:

(1) The amount of mode conversion and its frequency dependency increasesslightly as the thickness of member 12 is increased.

(2) The slope of phase angle line I-Sdecreases and then reverses as thethickness of member 12 is increased.

(3) Phase angle characteristics at the higher frequencies are virtuallyunaffected by the thickness of member 12..

The actual thickness chosen, therefore, is a matter of design choice.

What is claimed is:

1. A mode converter for converting energy in the TE11 mode to energy inthe TE11TM11 dual mode, said converter comprising,

rst and second circular waveguides joined together in coaxial alignmentfor electrical transmission therethrough, said rst waveguide having aninside diameter to support energy in the TE11 mode only and said secondwaveguide having an inside diameter to support energy in the TE11 andTM11 modes only, and

an annularly shaped dielectric member having an outside diametersubstantially equal to the inside diameter of said second waveguide andan inside diameter substantially equal to the inside diameter of saidrst waveguide and, furthermore, located in said second waveguideadjacent to said rst waveguide.

2. A mode converter for converting energy in the TE11 mode to energy inthe TE11TM11 dual mode, said converter comprising,

-- I a waveguidestructure having a iirst portion with an inside diameterto support energy in the TE11 mode only and a second portion with aninside diameter to support energy in the TEMo and TM11 modes only, and

5 an annularly shaped dielectric member having an outside diametersubstantially equal to the inside diameter of said second portion ofsaid waveguide structure and an inside diameter substantially equal tothe lo inside diameter of said first portion of said waveguide structureand, furthermore, located in said second portion of said waveguidestructure adjacent to said first portion of said waveguide structure.

15 References Cited UNITED STATES PATENTS 3,305,870 2/1967 Webb 333--21X 3,413,641 11/1968 Turrin 333-21 X 20 3,413,642 11/1968 Cook 343-781ELI LIEBERMAN, Primary Examiner MARVIN NUSSBAUM, Assistant Examiner 25U.S. Cl. X.R.

